Army can get stuck if it has too many resources, or you have an army camp set. So please check for that. But if it`s a rare bug that makes army get stuck on a mountain, there is no solution on our end - but there is something you can do on your end

Bring the army that`s stuck to the edge of the mountain, bring another army that can move normally on the worldmap, position it near that stuck army - and transfer units and resources from the stuck army into the normal army.